Unlike the hippo's ancestor, whale ancestors moved to the sea and evolved into swimming creatures over a period of about 8 million years. Fossils of gigantic ancient whales called Basilosaurus were first mistaken for dinasaur fossils but were later recognised as mammals.

What is the Basilosaurus related to?

Basilosaurus, also called Zeuglodon, extinct genus of primitive whales of the family Basilosauridae (suborder Archaeoceti) found in Middle and Late Eocene rocks in North America and northern Africa (the Eocene Epoch lasted from 55.8 million to 33.9 million years ago).

What did whales descend from?

Scientists since Darwin have known that whales are mammals whose ancestors walked on land, and in the past 15 years, researchers led by Dr. ... Thewissen have identified a series of intermediate fossils documenting whale's dramatic evolutionary transition from land to sea.